The wireless power transfer technology is a technology that wirelessly delivers power between a power source and an electronic device. For example, the wireless power transfer technology enables a battery of a wireless terminal to be charged by simply placing a wireless terminal such as a smart phone or a tablet on a wireless charging pad. Thus, compared to a wired charging environment using a typical wired charging connector, the mobility, convenience, and safety can be improved. In addition to wireless charging of wireless terminals, the wireless power transfer technology is attracting attention as a substitute for the existing wired power transfer environment in various fields such as electric vehicles, wearable devices such as Bluetooth earphones or 3D glasses, home appliances, furniture, underground facilities, buildings, medical devices, robots, and leisure.
The wireless power transfer method is also referred to as a contactless power transfer method, a no point of contact power transfer method, or a wireless charging method. The wireless power transfer system includes a wireless power transfer apparatus for supplying electric energy by a wireless power transfer method and a wireless power receiving apparatus for receiving electric energy wirelessly supplied from the wireless power transfer apparatus and supplying power to a power receiving device such as a battery cell.
The wireless power transfer technologies are largely classified into a magnetic induction method and a magnetic resonance method. In the magnetic induction method, energy is transmitted using a current induced at a receiving side coil due to a magnetic field generated in a coil battery cell at a transmitting side in accordance with electromagnetic coupling between a coil at the transmitting side and a coil at the receiving side. The magnetic induction type of wireless power transfer technology has an advantage of high transmission efficiency, but has limitations in that the power transfer distance is limited to several millimeters and the degrees of the location freedom is significantly low due to sensitivity to matching between coils.
The magnetic resonance method is similar to the magnetic induction method in that both methods use a magnetic field. However, in the magnetic resonance method, a resonance occurs when a specific resonance frequency is applied to the coil at the transmission side and the coil at the reception side, and thus energy is transferred by a phenomenon that the magnetic field is focused on both ends of the transmission side and the reception side, which differs from magnetic induction method in terms of energy transfer. Due to these characteristics of magnetic resonance, power can be remotely transmitted unlike magnetic induction. The magnetic resonance method may transmit energy up to a relatively long distance of several tens of centimeters to several meters compared to the magnetic induction method, and enables power transmission to a plurality of devices at the same time. Thus, the magnetic resonance method is expected to be a wireless power transfer technology to implement cord-free.